Wound dressing comprising silver sulfadiazine incorporated in animal tissue

ABSTRACT

Animal tissue and the like having silver sulfadiazine incorporated therein is useful as a covering for burns and/or wounds. The silver sulfadiazine is incorporated in the tissue by soaking the tissue in an ammoniacal silver sulfadiazine solution or suspension. When the silver sulfadiazine is so incorporated in the tissue via an ammonium solution, more silver is found to be present than would normally be expected and the effectiveness of the thus treated tissue as a wound covering increases.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing a dressing or coveringfor wounds and/or burns. Animal tissue has long been known to be auseful source of material for wound coverings. Although not identical tohuman skin, the composition and usefulness of some animal tissueresembles human skin. For example, a covering for wounds, ideally, bothprotects the wound against contact with moisture and externalcontaminants, while also being permeable to the excretions, inparticular, water vapor, which are produced by normal metabolicprocesses.

Silver sulfadiazine (AgSD) is known as a useful antimicrobial in thetreatment of wounds, and of burns in particular, see U.S. Pat. No.3,761,590 (1973), Fox; Bose, Storing Amniotic Membrane in SilverSulfadiazine Suspension, The Canadian Medical Assocation Journal, 127,No. 2, p. 112 (July 15, 1982); Fox, Modak and Stanford, Silver TreatedGraft Materials for Coverage of Infected Burn Wounds, Ann. Chir. Plast.24, N.3, 265-267 (1979).

While animal tissue, and pigskin in particular, satisfies certaincriteria for a burn wound covering, there are certain difficulties whichmilitate against using simple, untreated animal tissue as a woundcovering. First, and most important of these, is the fact that whileanimal tissue, when used as a wound covering, may protect the woundagainst external agents and influences, it does not and cannot preventinfection caused by bacteria, for example, which may have entered thewound prior to the application of the covering, or against whateverbacteria which may enter through the covering. While the problem may bealleviated in part by using a topical bacteriocide (e.g., silversulfadiazine cream) applied to the skin covering, there is still achance of infection entering the wound when the dressing must bechanged. Such changes must be done with some frequency as the woundcover, even though permeable, will tend to become sodden from topicalbacteriocides as well as from metabolic waste products. These changes ofdressing allow bacteria to enter, even if the wound is left uncoveredfor a very short time. Also, while animal tissue is carefully treated soas to attempt to insure that the tissue is initially free of bacteria,sterility cannot be guaranteed, particularly when applied as a woundcovering. In particular, should the wound be allowed to be exposed toair, the chance of infection by airborne bacteria is made even greater.

SUMMARY OF THE INVENTION

It has been found that ammoniated or aqueous ammoniated silversulfadiazine solution incorporated in animal tissue provides aneffective dressing for burns and/or wounds. The method of incorporationof silver sulfadiazine in animal tissue may be accomplished in severalways, as described herein. After incorporation of ammoniated silversulfadiazine with animal tissue takes place, analysis reveals that moresilver is incorporated than would be expected. It is thought that theexcess incorporation of silver is a result of incorporation of silverions with collagen molecules within the tissue, although this isuncertain, and the inventors do not bind themselves to this theory.

DETAILED DESCRIPTION OF THE INVENTION

Samples of tissue taken from slaughtered animals are first prepared forlater treatment for use as wound dressings. The samples of tissue areselected from redently slaughtered, healthy animals. Preferably, thetissue is taken from animals which have been slaughtered no more thanone year prior to tissue selection. Preferred tissues include animalskin, for example, porcine tissue or pigskin, and human ammiotic tissue.The porcine tissue is removed via use of clean, sharp knives, and theremoved tissue is then placed immediately in cooling containers, and isthen transported to a processing laboratory. At such a processinglaboratory, the samples are cut to sizes suitable for use as covers forparticular wounds and the samples are then ready for the next phase intheir preparation as wound dressings.

As different types of injury require different sizes and shapes ofdressings, the cleaned tissue is cut to appropriate sizes and shapes.The tissue is then prepared for treatment.

Samples are placed in containers which contain solutions of normalphysiological saline. The containers are heat sealed, and thenirradiated with either beta or gamma radiation. Such radiation treatmenthas the effect of both sterilizing and tanning the material. Followingradiation treatment, the samples are placed in quarantine freezerstorage and then subject to culturing procedures to determine sterility.The culturing procedure involves immersing the tissue in culture mediumwhich has been chosen so as to be conducive to bacterial growth. Thetissue is observed for growth of bacteria, and if such growth appears atthe end of 14 days or less, the tissue is discarded as not sterile. Ifsuch growth is not observed, the tissue is considered sterile, and isdeemed suitable for treatment with sulfadiazine and solutions containingalso silver in the presence of ammonium ions.

The samples of sterile animal tissue may then, for example, be placed inan aqueous solution containing ammoniacal silver sulfadiazine. Thesource of the ammonium ions may vary, although one preferred source ofthe ammonium ions is NH₄ OH in solution. The samples are placed insolution for the desired length of time, and then removed and allowed topartially dry. Complete drying is not desirable, as such complete dryingmay result in shrinking, hardening, and cracking of the samples,rendering them unsuitable for use in burn and wound therapy. Followingpartial drying, the samples are placed in storage under sterileconditions.

Soaking is not the only method of incorporation of ammoniated silversulfadiazine in the animal tissue. For example, it is possible to spraypreviously prepared samples of animal tissue with ammoniated solutionsof silver sulfadiazine. The contacting of the ammoniated silversulfadiazine to the animal tissue results in incorporation within thetissue matrix as previously described, and the subsequent method ofstorage and handling is identical.

In use as a wound covering, the ammoniated silver sulfadiazineincorporated animal tissue is directly applied to the burn or wound, andallowed to remain in contact with the burn or wound surface for thecourse of the therapy. The treatment of wounds or burns using such woundcovers or dressings results in greater protection and survival againstbacterial infection.

The following examples point out with more specificity the practices andadvantages of the practices of the invention.

EXAMPLE I

8×10 cm. patches of pigskin were soaked in 100 ml. of 10 mol/mlsolutions of sodium sulfadiazine. The patches are soaked for 4 days(shorter exposure to sodium sulfadiazine tended to yield a lesseffective product), after which the patches are blotted dry on 4×4 gauzesquares. The patches are then placed in 100 ml of 5 mol/ml solutions ofAgNO₃ for 10 minutes. After such treatment in AgNO₃ solution, thepigskin was washed four times with distilled water.

In order to determine the amount of sulfadiazine and Ag⁺ present in thetissue samples, assays employing standard techniques are used.Sulfadiazine content was determined by the method of Bratton & Marshall,and Ag⁺ content is determined by performing red pyrogallol colorimetry.Such assays revealed that 0.5 mol/cm² of sulfadiazine was available in apigskin sample, while 0.83 mol/cm² of Ag⁺ was available. This suggeststhat some silver becomes bound to protein molecules in the pigskin, inaddition to forming silver sulfadiazine compounds.

The protein molecules to which the silver is bound are, presumably,collagen, see Fox, Modak and Stanford, "Silver Treated Graft Materialsfor Covering of Infected Burn Wounds," J. Ann. Chir. Plast. 1979, 24,pp. 265-67.

EXAMPLE II

The efficacy of pigskin samples having silver sulfadiazine incorporatedtherein was tested, and compared to a standard wound dressing containingAg⁺ in the absence of sulfadiazine.

Samples of pigskin were prepared according to the method set forth inExample I. Discs of treated pigskin measuring 1 cm. in diameter were cutfrom such samples, and placed upon culture media containing colonies ofPseudomonas aeruginosa. It was observed that the samples of pigskinwhich contain silver sulfadiazine produce a zone of inhibition on suchcultures measuring 16-17 mm. in diameter. In comparison, thecommercially available pigskin wound covering known as MEDISKIN (productof Genetic Laboratories, Inc.), incorporated with silver but not silversulfadiazine, produced a zone of inhibition of 12-13 mm. under identicalconditions.

EXAMPLE III

In vivo experiments were performed to determine the efficacy of pigskinwith incorporated silver sulfadiazine in accordance with the inventionand compared with other wound covers.

Laboratory mice were divided into three groups, each containing fivemice, and contact burns were caused by applying a heat source (hotplate) of 150° C. for five seconds to dorsa which had been shaven withan electric clipper. Burn eschar which formed was removed one hour postburn, and the wounds were seeded with a virulent strain of Pseudomanasaeruginosa (Boston) bacteria. The infections were 0.1 ml of 0.6 O.D. at600 NM innoculum. The wounds were then covered with one of threedressings: plain MEDISKIN (product of Genetic Laboratories, Inc.),MEDISKIN incorporated with silver, or pigskin with which has beenincorporated silver sulfadiazine. The criterion observed was survival.Results are summarized in the following Table I:

                  TABLE I                                                         ______________________________________                                        A. Pigskin  B. MEDISKIN   C. Plain                                            + AgSD      + Ag.sup.+    MEDISKIN                                                  Mortality                                                               Days  Rate (#   Days    Mortality                                                                             Days  Mortality                               After and per-  After   (# and  After Rate (#                                 Burn  centage)  Burn    Percentage)                                                                           Burn  Percentage)                             ______________________________________                                        1     0     0%      1     2    40%  1     2    40%                            2     0     0%      2     5   100%  2     5   100%                            3     0     0%      3     5   100%  3     5   100%                            4     1     20%     4     5   100%  4     5   100%                            5     1     20%     5     5   100%  5     5   100%                            ______________________________________                                    

EXAMPLE IV

A second comparison of plain MEDISKIN and pigskin with which silversulfadiazine is incorporated was performed. The group contained threemice each, and conditions were the same as those described in ExampleIII. Results of this experiment are set out in TABLE II:

                  TABLE II                                                        ______________________________________                                        Pigskin         Plain                                                         + AgSD          MEDISKIN                                                      Days                Days                                                      After   Mortality   After        Mortality                                    Burn    (% and #)   Burn         (% and #)                                    ______________________________________                                        1        0%      0      1        0%  0                                        2        0%      0      2       33%  1                                        3        0%      0      3       67%  2                                        4        0%      0      4       67%  2                                        5        0%      0      5       67%  2                                        6        0%      0      6       67%  2                                        7       33%      1      7       67%  2                                        8       33%      1      8       67%  2                                        9       33%      1      9       67%  2                                        ______________________________________                                    

EXAMPLE V

In vivo observation of the bacteriocidal properties of AgNO₃ soakedpigskin and pigskin with incorporated silver sulfadiazine was observed.

Fifteen female rats received burn wounds from a metal hot plate appliedto shaved dorsa, producing wounds measuring approximately 4×8 cm.Approximately 24 hours later wound eschars were removed and the woundswere seeded with 0.5 ml samples of Pseudomonas aeurigimosa (Boston)bacteria, of 0.60 O.D. One hour later, wound covers were applied to 12of the mice: three of the mice had wounds covered with pigskin which hadbeen soaked in 0.01 M AgNO₃ for one hour; a second group received woundcovers of pigskin soaked in 0.01 M silver sulfadiazine solution for onehour; the third group's wounds were covered with plain pigskin which hadnot been soaked in any solution; a fourth group received covers ofpigskin soaked in distilled water, and three rats in the fifth group hadwounds left opened. Those rats which received no wound cover and thosewhich received wound covers soaked in water served as controls forinitial infection. The rats were observed from day to day, and thoserats which survived more than nine days, when dressings were changed,were observed for infection. The results of these three groups (plainpigskin, AgNO₃ pigskin, and silver sulfadiazine pigskin) follow:

    __________________________________________________________________________    SUBJECT                                                                       Material                                                                             #1        #2        #3                                                 __________________________________________________________________________    Plain Pigskin                                                                        Died: Pseudomonas-                                                                      Died: Pseudomonas-                                                                      Died: 13 day                                              5 days    6 days    (Pseudomonas                                                                  suspected)                                         AgNO.sub.3                                                                           Died: Proteus-                                                                          Died: Pseudomonas-                                                                      Severe Pseudomona                                  Pigskin                                                                              5 days    6 days    infection                                          Silver Sulfa-                                                                        Mild Proteus                                                                            Mild Proteus                                                                            No infection                                       diazine                                                                              infection infection                                                    pigskin                                                                       __________________________________________________________________________

No Pseudomonas infection was discovered in any of the three mice treatedwith silver sulfadiazine soaked pigskin. Mild proteus infections werediscovered in two of them, but the infections were not severe enough tocause mortality. Infection by Pseudomonas was also observed in thoserats which had been treated with pigskin soaked in water.

EXAMPLE VI

The efficacy of pigskin with incorporated silver sulfadiazine in ammoniasolutions was tested. Samples of pigskin with incorporated silversulfadiazine in ammonia solutions as described in Example I areprepared, and tested for antibacterial activity. The test involved theuse of standard circular disks which were cut from the samples, andplaced upon blood agar plate cultures infected with Pseudomonasaerugenosa. Comparison tests were run with circular disks of the samesize, which had been impregnated with a combination of sodiumsulfadiazine and silver nitrate (NaSD+AgNO₃), ammonium hydroxide and, asa control, pigskin samples with no impregnated salts. The results aretabulated in Table I.

                  TABLE I                                                         ______________________________________                                        In Vitro Efficacy of Wound Covers Impreg-                                     nated with Various Substances                                                 Substance        Zone of Inhibition                                           ______________________________________                                        NaSD + AgNO.sub.3                                                                              18 mm.                                                       AgSD in ammoniated                                                                             24 mm.                                                       solution                                                                      NH.sub.4 OH       0 mm.                                                       None              0 mm.                                                   

The results clearly indicate the superiority of the pigskin tissue inwhich silver sulfadiazine in ammoniated solution is incorporated withthe pigskin.

EXAMPLE VII

The in vivo efficacy of pigskin incorporated with ammoniated silversulfadiazine was examined.

Swiss mice were anesthetized, and 2×3 cm areas of dorsa were burned onhot plates at 150° C. followed by excision of the burn eschar andinfection with 10⁷ concentrations of Pseudomonas aeruginosa. The micewere divided into groups, and received burn wound covers as follows:untreated pigskin, and pigskin which had been soaked in a solution ofammoniated silver sulfadiazine, as detailed in Example I. Additionally,a third group of mice received applications of silver sulfadiazine creamwhich had not been prepared in ammoniated solution. The criterionobserved was survival--i.e., the mice were observed at two-dayintervals, and the percentage of mortality was calculated with theresults tabulated as follows.

                  TABLE II                                                        ______________________________________                                        In Vivo Efficacy of Silver Sulfadiazine                                                     % Mortality                                                     Wound Cover     2 days    4 days  6 days                                      ______________________________________                                        Untreated pigskin                                                                             100       100     100                                         Pigskin treated with                                                                           0         0       33                                         silver sulfadiazine                                                           in ammonium hydroxide                                                         ______________________________________                                    

Those mice which received non-ammoniated silver sulfadiazine received itin the form of 1% silver sulfadiazine cream administered to the top ofuntreated pigskin wound covers. Such mice showed a mortality rate of 80%over the same six-day period.

EXAMPLE VIII

To test the in vivo efficacy of pigskin with ammoniated silversulfadiazine incorporated therein, animal experiments were performed.

Female Swiss mice were anaesthetized, and 2×3 cm. areas of dorsal skinwere burned at 150° C., using a hotplate. After one hour, 1.5 cm² ofburn eschar was removed from each wound, and 0.03 D cultures ofPseudomonas aeruginosa were swabbed into the wounds. After an incubationperiod of one hour, pieces of pigskin measuring 2×3 cm. were used tocover the wounds. This pigskin had either been treated with ammoniatedsilver sulfadiazine, silver sulfadiazine cream (silvadene), or was leftuntreated. The criteria observed over a period of six days was survival.The relevant data is summarized below.

    ______________________________________                                                  % Mortality (Days Postburn)                                         Pigskin     1       2     3      4    5     6                                 ______________________________________                                        Untreated   0       60    100    100  100   100                               Ag SD cream 0        0     50     50  100   100                               Ammoniated Ag SD                                                                          0        0     0      0    10    10                               ______________________________________                                    

EXAMPLE IX

Additional in vitro experiments were performed to determine the zone ofinhibition of pigskin with incorporated ammoniated silver sulfadiazine.Standard circular disks were cut from impregnated dressings and testedfor antibaceterial activity against Pseudomonas aeruginosa on blood agarplate cultures. As controls, identical circular disks with AgNo₃ and NH₄OH were prepared. The zones of inhibition are compared below:

    ______________________________________                                                        Zone:                                                         ______________________________________                                        Ammoniated AgSD   30 mm.                                                      AgNO.sub.3        12 mm.                                                      NH.sub.4 OH        0 mm.                                                      ______________________________________                                    

EXAMPLE X

The incorporation of silver sulfadiazine in homografts, and itssubsequent retention was tested.

Human skin homografts were cut into 2 cm² pieces, and then soaked in 4%ammoniacal, ¹¹⁰ AGSD for four hours. Following soaking, the homograftsamples were dried at room temperature, in the dark, until the ammoniawas driven off.

Pieces were then divided into two groups, one of which was washed fivetimes with water, and the other remained untreated. The specimens werethen tested, for levels of incorporation of silver sulfadiazine, andzones of Inhibition. The results are summarized below:

    ______________________________________                                                                Zone of                                                              Drug Level                                                                             Inhibition                                            ______________________________________                                        Washed homograft 0.21 mole/mg                                                                             25 mm.                                            Unwashed homograft                                                                             0.24 mole/mg                                                                             25 mm.                                            ______________________________________                                    

What is claimed is:
 1. A method for preparing animal tissues for use asburn or wound dressings which comprise soaking animal tissue in anaqueous solution of ammoniated silver sulfadiazine to incorporate theammoniated silver sulfadiazine to the animal tissue, and partiallydrying the thus-treated animal tissue.
 2. A method as described in claim1, wherein the animal tissue is sprayed with aqueous ammoniated silversulfadiazine solution.
 3. A method as described in claim 1, wherein theanimal tissue is pigskin.
 4. A method as described in claim 1, whereinthe animal tissue is amniotic tissue.
 5. A method as described in claim1, wherein said aqueous solution of ammoniated silver sulfadiazinecontains ammonium hydroxide.
 6. The product of the method described inclaim 1, wherein silver sulfadiazine has been incorporated.
 7. A methodfor the treatment of burns and/or wounds, such method comprising theapplication of the product of claim 1 to wounds and/or burns.
 8. Theproduct of the method of claim 1, wherein said animal tissue is human orcadaver skin.
 9. A method as described in claim 1, wherein said tissueis human skin.